sinon shorter stack errors, add in assert.fail

gistfile1.js

fail: function fail(message) {
    var error = new Error(message);
    error.name = this.failException || assert.failException;

    try{
      throw error;
    } catch(e) {
        if (error.stack){
            console.log(error.stack);
            error.stack = error.stack.split('\n').slice(0,20).join('\n')
        }
        throw e;
    }
},